Method and apparatus for electrically perforating sheet or web material

ABSTRACT

A method and apparatus for perforating a web or sheet of material by passing a plurality of sparks through the web, with the first spark forming the perforation and the remaining sparks enlarging the perforation, and controlling the number of sparks to control the size of the perforation.

United States Patent inventors Rene Larlve;

Richard L. C. Knight, both of GrandMere, Quebec, Canada June 12, 1969Nov. 23, 1971 Consolidated Paper (Bahamas) Limited Nassau, Bahamas Appl.No. Filed Patented Assignee METHOD AND APPARATUS F OR ELECTRICALLYPERFORATING SHEET OR WEB MATERIAL 13 Claims, 7 Drawing Figs.

us. ca 219/384, 8 3/3 65 Int. Cl 1105b 7/18 Field of Search..219/383-384,

[56] References Cited UNITED STATES PATENTS 2,528,158 10/1950 Menke3,020,377 2/1962 Shawfrank 3,385,951 5/1968 Bancroft et a1. 3,419,75212/1968 Schmidt 3,167,641 1/1965 Parmela et a1.. 3,351,740 11/1967 Heuer3,371,190 2/1968 Meyer..... 3,475,591 10/1969 Fujii et a1 PrimaryExaminer-Volodymyr Y. Mayewsky Attorney-Alan Swabey ABSTRACT: A methodand apparatus for perforating a web or sheet of material by passing aplurality of sparks through the web, with the first spark forming theperforation and the remaining sparks enlarging the perforation, andcontrolling the number of sparks to control the size of the perforation.

L/ ii magne ic METHOD AND APPARATUS FOR ELECTRICALLY PERFORATING SHEETOR WEB MATERIAL BACKGROUND OF INVENTION 1 Field of Invention Thisinvention relates to an improved method and apparatus for electricallyperforating a sheet or web or material.

More particularly the invention relates to an improved method andapparatus for electrically perforating a moving web or sheet whereby thesize of the perforations can be readily controlled.

2. Description of the Prior Art Electrical perforating of webs or sheetsby sparks is well known. However, the perforations are generally of onesize only. The size of the perforations could be varied by increasing.the size of the single spark .used to form the perforations. However,it has been found extremely difficult in practice to do this.

Mechanical perforating of webs is also well known whereby the web ispassed between mating rollers with one roller having pins cooperatingwith grooves in the other roller to punch perforations in the web. Thisapparatus has disadvantages in that the size of perforations cannot bevaried unless the rollers are replaced. The pins become worn and dullrapidly thus requiring frequent replacement and servicing of the rolls.The pins also tend to bend and thus, do not perforate properly. Theperforating operation frequently raises the surface of the web adjacentthe perforation formed, thus forming a rough finish to the web surface.Additionally, when the pins are worn slightly, the perforation is notalways completely formed, resulting in the formation of a flap which canclose the perforation.

It is the purpose of the present invention to overcome the prior artdisadvantages and provide a method and apparatus for perforating webs orsheets which can be easily controlled to form perforations of varyingsize, which does not mar the surface appearance of the perforated webs,which cleanly forms the perforations, and which is simple and reliablein operation.

SUMMARY OF INVENTION The invention is particularlydirected toward amethod and apparatus for electrically perforating a sheet or web wherethe sheet or web is relatively moved between a pair of opposedelectrodes and two ormore electrical discharges of a current are passedbetween'the electrodes through the sheet or web, the first dischargeperforating the sheet and each succeeding discharge enlarging theperforation, and controlling the number of discharges to control thesize of the perforation.

Theapparatus and method are particularly suitable for perforating layersor laminations of plastic and paper webs or sheets, used inmanufacturing bags. The perforations make the bags porous or airpermeable and the porosity can be readily controlled merely bycontrolling the size of the perforations without having to change thenumber of perforations made or speed of operation of the perforatingapparatus. Paper or plastic webs alone could be perforated if desired.

Polyethylene and other similar plastic materials, when produced as thinsheets or webs, are substantially impervious to the passage of moisture.This characteristic of polyethylene has led to it use as a coatingmaterial for kraft paper, for example, and more particularl for coatingkraft paper used'in manufacturing multiwalled cement bags where it isimportant to keep the cement in a dry condition.

Prior to the use of the polyethylene-coated material for cement bags,the kraft paper in its uncoated condition had a porosity of about 400ml./min. over a sample area of 5 cm. with a pressure drop of l50 mm. ofwater (as measured with a Bendtsen porosity tester) which was adequateto permit the escape of trapped air through the bag wall as the bag wasbeing filled with cement. With the advent of polyethylenecoated kraftpaper for cement bags, it was found that although the polyethyleneprovideda substantial moisture barrier" to keep the cement dry, it wasso impervious that it prevented the passage of air therethrough, unlikeuncoated bags; and accordingly, resulted in bursting the bags duringfilling or alternatively, required slow filling of the bags.

In order to overcome the undesirable characteristics ofpolyethylene-coated kraft paper, the coated paper was minutely,mechanically perforated in a manner to permit the escape of air whilestillforming a suitable moisture barrier.

However, mechanical perforating has the disadvantages previouslyreferred to. The present apparatus has been found to suitably perforatethe bag material with minute perforations to provide the necessaryporosity while still providing a material which acts as a suitablemoisture barrier. BRIEF DESCRIPTION OF THE DRAWINGS The invention willnow be described in detail having reference to the accompany drawings inwhich: FIG. 1 is a diagrammatic circuit view showing a preferredembodiment of the perforating apparatus; and

FIG. 2A to FIG. 2F illustrate the waveforms various portions of theapparatus DESCRIPTION OF PREFERRED EMBODIMENT The apparatus forperforating a web or sheet W of material includes 1 or more sets 3 ofspaced-apart electrodes 1, 2. .As shown in FIG. 1, the electrodes 1, 2of each set 3 are spaced apart across the width of the web W to providea gap through which the web W is moved. The electrodes 1, 2 are pointelectrodes although, preferably, one of the electrodes will be a lineelectrode. The electrodes 1, 2 of each set 3 are connected in series tothe secondary 5 of a high-voltage transformer 4. The high-voltagetransformer 4 can comprise an ignition coil similar to that used inautomobiles.

A high-frequency alternating voltage is applied to the primary 6 of thetransformer 4 from a voltage source 7. The voltage source 7 can comprisemotor-generator set. The frequency of the motor-generator set normallycan range from 400 Hz. to 10 kHz., although a wider range can be usedwhen perforating webs running at very low or high speeds.

A control circuit 8 is provided to control the application of thehigh-frequency alternating voltage, the waveform of which is shown ant,to the primary 6 of transfonner 4 and thus, the number of sparks passingbetween each pair of electrodes 1, 2. The control circuit 8 preferablyincludes a pair of oppositely conducting silicon-controlled rectifiers9, 10 arranged in back-to-back configuration. Each SCR 9, 10 conducts onalternative half-cycles of the alternating voltage as shown by waveformsB and C. Each time one of SCRs conducts, a spark is passed between eachpair of electrodes 1, 2. To control the number of sparks discharged, thelength of time that the SCRs 9, 10 are capable of conducting iscontrolled. This is done by applyingsquare wave control pulse ofconstant width, as shown by D, to the gates of the SCRs, to make themconducting. The width of the control pulse controls the number of sparksdischarged between the electrodes. The control pulse is produced by aconventional pulse-shaping circuit 12 and fed to a primary 13 of atransfonner l4, and through secondaries l5, l6 to.tl1e gate of eachrespective SCR 9, 10 connected thereto.

For a webW travelling between the sets of electrodes 1, 2 at constantspeed, the pulse-shaping circuit can be set up to provide a controlpulse of predetermined constant width and at a predetermined frequencywhich will, for a fixed frequency from the voltage source 7, causeapredetermined number of sparks to pass between the electrodes. Thefirst spark perforates the web, and because the time for thepredetermined number of sparks to occur is relatively much less than thespeed of the web, each succeeding spark of the predetermined number willpass through the perforation to enlarge it. Thus, the size of eachperforation can be easily controlled through the pulse-shaping circuit.

While the size of the perforations can be simply controlled bycontrolling the width of the control pulse as described, the

occurring in size can also be controlled by means of the voltage source(in changing its frequency or voltage), either alone or in conjunctionwith the use of the control circuit.

If desired, the perforating operation can be directly controlled by thespeed of travel of the web. As shown in FIG. 1, an actuating circuit 17is connected to operate the pulse-shaping circuit 12. The actuatingcircuit can comprise a toothed steel wheel 18 connected to a rotatingshaft of the web-feeding apparatus. A magnetic pickup 19 is locatedadjacent the periphery of the wheel 18 and every time a tooth 20 of thewheel 18 passes the pickup 19 during rotation of the wheel, a pulse E isgenerated to control the operation of the shaping circuit 12. Thefrequency of the control pulse is thus controlled by the web speed. Thisarrangement is useful where the speed of the web varies during itspassage between the electrodes. The arrangement permits the web to beuniformly perforated despite variation in its speed.

Other actuating means to control the operation of the pulseshapingcircuit can be used. For example, for relatively slow web speeds, anoptical sensing means can be used with the toothed wheel instead of themagnetic pickup to provide pulses controlling the pulse-shaping circuit.

The operation of the apparatus having reference to both FIG. I and FIGS.2A-2F shows diagrammatically the time relationship of the waveformsillustrated in FIG. 1. The output pulse E of the magnetic pickup 19 isshown in FIG. 2A. The output pulse E is fed to the pulse-shaping circuit12 which provides control pulses D of constant predetennined width WP asshown in FIG. 2B. These pulses D are the input to the gates of the SCRs9, 10. FIG. 2C shows the alternating voltage output A from the voltagesource 7 having voltage half-cycles A and A Upon the application of theinput pulses D to the SCRs, the SCRs are made conducting on theirrespective forward biased half-cycles and each conducts or passes anumber voltage half-cycles A and A respectively, depending on the widthof control pulse D as shown in FIGS. 2D and 2E to provide outputs B andC, respectively. Each of the half-cycles A A are transfonner coupled tothe electrodes to produce a spark F for each set as shown in FIG. 2F.

The number of sparks F in each group or burst can be easily controlledby varying the control pulse width WP. The first spark F, passingbetween the electrodes 1, 2 forms the perforation in the moving web W,and each succeeding spark F F of the group, taking the path of leastresistance, passes through the perforation in the web to enlarge it. Theburst of sparks occurs within a relatively very short time and the webhas moved a short distance within this allowing all the sparks F -F topass through the perforation.

If a large series of sparks are supplied in a single burst, theapproximate first half of the series of sparks can form and enlarge afirst perforation. This occurs as a result of the web moving relative tothe electrodes. The sparks between the electrodes take a path of leastresistance and as the first perforation moves away from the electrodes,the sparks follow the perforation until the distance of the perforationfrom the electrodes becomes so great that the electrical resistance ofthe unperforated web located between the electrodes is less than theairpath through the perforation which is moving away from theelectrodes. Thus, the remainder of the sparks of the burst form a secondperforation and enlarge it as well.

The apparatus, by way of example, can comprise a 3-kl-lz. 300-v.motor-generator set which is thus capable of producing 6,000 sparks/sec.if desired. With a web travelling at 300 feet/min. and a desiredperforation spacing of one-half inch, the maximum time available foreach burst of sparks is 4.1 milliseconds, and thus a burst will containapproximately 25 individual sparks. For any desired porosity, the numberof sparks per hole depends on the speed of operations. Using a 24-v.ignition coil as the transformer 4, the 300-v. motorgenerator set canprovide a voltage of approximately 25 kv. at the secondary of thetransformer which is sufficient to produce a spark 1 inch long. By usinga series of electrodes with small gaps, several sparks can be producedfrom a single transformer. For example, 12 electrode sets may be used toproduce 12 rows of holes oil the same transformer, A plurality oftransformers and electrode sets can be arranged to extend across thewidth of the web or sheet. The holes produced are minute and barelyvisible to the naked eye. For example, to obtain a Bendtsen porosity ofml./min./in. in a web running at up to 400 ft./min. with one-half-in.hole spacing, the holes produced are 0.0055 inch in diameter.

The webs being perforated are preferably polyethylene (or similarplastic)-coated Kraft paper although other materials having similardielectric qualities can likewise be perforated. Additionally, althoughthe particular problem of maintaining a moisture barrier occurs in thecement bags, perforations of this character can also be used, forexample, on glue flaps of various web or sheet materials in order topermit the moisture to evaporate and enable the glue to dry properly.Depending on the voltage source and type of web, several layers ofplastic or plastic-coated papers can be perforated. For example, using a300-v. motor-generator set, plastic material up to a thickness of 1 milor plastic-coated paper with a plastic film less than 1 mil thick can beperforated.

Although a 24-v. ignition coil is suitable for use as transformer 4,other types of high-voltage coils could be used, such as for example, a6-or l2-v. ignition coil.

We claim:

1. A method of perforating a web comprising the steps of:

relatively moving the web in a path of travel between a gap defined by aplurality of pairs of electrodes:

at least one electrode of each said pair being a point electrode:

providing a high-frequency alternating voltage to each said plurality ofpairs of electrodes; passing an electrical discharge between each pairof electrodes of said plurality of pairs of electrodes and thus throughthe web whereby the first discharge between each pair of electrodesforms a perforation in the web:

passing at least one further electrical discharge across each pair ofsaid plurality of pairs of electrodes in the are of said perforationthereby to enlarge said perforation immediately after said perforationis formed:

and adjustably controlling the number of said further electricaldischarges enlarging the perforation thereby to control the size ofperforation formed.

2. A method as claimed in claim 1, including controlling the number ofperforations produced in relation to the relative rate of travel of theweb with respect to the electrodes.

3. A method as defined in claim 1, including the step of providing asource of alternating voltage, causing said pairs of electrodes todischarge on every half-cycle of said alternating voltage, andadjustably controlling the number of said half-cycles thereby to controlthe number of said further electrical discharges.

4. A method as claimed in claim 3, including providing a series ofcontrol pulses of predetermined width, each control pulse correspondingto a half-cycle of said alternating voltage.

5. A method as claimed in claim 4, wherein the frequency of the controlpulses is a function of web speed.

6. An apparatus for perforating a web comprising means for relativelymoving the web in a path of travel between a gap defined by a pluralityof pairs of electrodes, at least one electrode of each said pair ofelectrodes being a point electrode, a source of high-frequencyelectrical pulses electrically connected to said electrodes and havingsufficient potential to cause electrical discharges to pass between eachpair of electrodes and through the web whereby the first dischargebetween each pair of electrodes forms a perforation in the web and eachsucceeding discharge across each pair of electrodes enlarges theperforation formed, electronic control means for adjustably controllingthe number of electrical discharges enlarging the perforation to controlthe size of the perforation formed.

7. An apparatus as claimed in claim 6, wherein said source is analternating voltage source.

8. An apparatus as claimed in claim 7, wherein the adjustable controlmeans includes electronic gate means controlling the passage of apredetermined number of half-cycles of alternating voltage from saidalternating voltage source.

9. An apparatus as claimed in claim 8, wherein said electronic gatemeans controlling the passage of the half-cycles includes two oppositelyconducting controlled rectifiers conducting respectively on the positiveand negative peaks of each voltage cycle.

10. An apparatus as claimed in claim 9, wherein said electronic controlmeans further includes means for generating a series of control pulses,said series of control pulses being ap plied to said control rectifiersto govern the conducting period for said control rectifiers.

11. An apparatus as claimed in claim 15, including means controlling thefrequency of the control pulses as a function of web speed.

12. A circuit for use in a system to perforate a web comprisingrate-of-movement-sensing means for generating a control pulse inrelation to the rate of movement of said web;

pulse-shaping means connected to said rate-of-movementsensing means forgenerating a squared waveform control pulse of a predetermined width;

coupling transformer means connected to said ing means for generating acontrol current; current rectifier means having control gate meansconnected to said coupling transformer means; alternating currentgenerating means connected to said current rectifier means; and

current discharge means connected to said current-generating means andsaid current rectifier means whereby periodic generation of a shapedcontrol pulse results in the generation of a controlled number of staticdischarges the first of said controlled number of static dischargesperforating said web and the remaining number of said static dischargesenlarging said perforation.

13. In a circuit as set forth in claim 12, in which said currentrectifier means comprises a pair of controlled rectifiers inback-to-back configuration for alternately generating a static dischargeduring each half-cycle of alternating current when made conducting bythe control pulse.

pulse shap-

1. A method of perforating a web comprising the steps of: relativelymoving the web in a path of travel between a gap defined by a pluralityof pairs of electrodes; at least one electrode of each said pair being apoint electrode; providing a high-frequency alternating voltage to eachsaid plurality of pairs of electrodes; passing an electrical dischargebetween each pair of electrodes of said plurality of pairs of electrodesand thus through the web whereby the first discharge between each pairof electrodes forms a perforation in the web; passing at least onefurther electrical discharge across each pair of said plurality of pairsof electrodes in the area of said perforation thereby to enlarge saidperforation immediately after said perforation is formed; and adjustablycontrolling the number of said further electrical discharges enlargingthe perforation thereby to control the size of perforation formed.
 2. Amethod as claimed in claim 1, including controlling the number ofperforations produced in relation to the relative rate of travel of theweb with respect to the electrodes.
 3. A method as defined in claim 1,including the step of providing a source of alternating voltage, causingsaid pairs of electrodes to discharge on every half-cycle of saidalternating voltage, and adjustably controlling the number of saidhalf-cycles thereby to control the number of said further electricaldischarges.
 4. A method as claimed in claim 3, including providing aseries of control pulses of predetermined width, each control pulsecorresponding to a half-cycle of said alternating voltage.
 5. A methodas claimed in claim 4, wherein the frequency of the control pulses is afunction of web speed.
 6. An apparatus for perforating a web comprisingmeans for relatively moving the web in a path of travel between a gapdefined by a plurality of pairs of electrodes, at least one electrode ofeach said pair of electrodes being a point electrode, a source ofhigh-frequency electrical pulses electrically connected to saidelectrodes and having sufficient potential to cause electricaldischarges to pass between each pair of electrodes and through the webwhereby the first discharge between each pair of electrodes forms aperforation in the web and each succeeding discharge across each pair ofelectrodes enlarges the perforation formed, electronic control means foradjustably controlling the number of electrical discharges enlarging theperforation to control the size of the perforation formed.
 7. Anapparatus as claimed in claim 6, wherein said source is an alternatingvoltage source.
 8. An apparatus as claimed in claim 7, wherein theadjustable control means includes electronic gate means controlling thepassage of a predetermined number of half-cycles of alternating voltagefrom said alternating voltage source.
 9. An apparatus as claimed inclaim 8, wherein said electronic gate means controlling the passage ofthe half-cycles includes two oppositely conducting controlled rectifiersconducting respectively on the positive and negative peaks of eachvoltage cycle.
 10. An apparatus as claimed in claim 9, wherein saidelectronic control means further includes means for generating a seriesof control pulses, said series of control pulses being applied to saidcontrol rectifiers to govern the conducting period for said controlrectifiers.
 11. An apparatus as claimed in claim 15, including meanscontrolling the frequency of the control pulses as a function of webspeed.
 12. A circuit for use in a system to perforate a web comprisingrate-of-movement-sensing means for generating a control pulse inrelation to the rate of movement of said web; pulse-shaping meansconnected to said rate-of-movement-sensing means for generating asquared waveform control pulse of a predetermined width; couplingtransformer means connected to said pulse-shaping means for generating acontrol current; current rectifier means having control gate meansconnected to said coupling transformer means; alternating currentgenerating means connected to said current rectifier means; and currentdischarge means connected to said current-generating means and saidcurrent rectifier means whereby periodic generation of a shaped controlpulse results in the generation of a controlled number of staticdischarges the first of said controlled number of static dischargesperforating said web and the remaining number of said static dischargesenlarging said perforation.
 13. In a circuit as set forth in claim 12,in which said current rectifier means comprises a pair of controlledrectifiers in back-to-back configuration for alternately generating astatic discharge during each half-cycle of alternating current when madeconducting by the control pulse.